I. Field of the Invention
This invention relates generally to cardiac pacer apparatus and more specifically to an improved cardiac pacer system which is responsive to the metabolic needs of the body and which optimizes cardiac output (blood flow) to suit those metabolic needs.
II. Discussion of the Prior Art
The human body can be viewed as a machine that is used by the mind to allow it to function in the physical world. It is started at conception and continues running until death, and like any machine, it requires energy to operate. The amount of energy required varies from "idle" when sleeping to "full throttle" during periods of maximum exertion and this energy is supplied through the blood stream. The heart, which is the pump for this system, must maintain this flow of energy so as to equal that required by the body during any given condition, such as during periods of sleep or during exercise.
In a normal heart, cardiac demand controls the cardiac output (blood flow rate) by virtue of the conduction of electrical impulses from the SA node to the AV node and from there down the bundle of His which passes to the ventricular septum, where it divides into two large bundle branches which supply both ventricles. Each branch spreads along the endocardial surface of the septum to the apex of the heart and then turns upward over the lateral wall of each ventrical. The Purkinje fibers of the AV bundle enter the ventricular walls and finally fuse with the heart muscle fibers and, in this way, each muscle fiber receives the impulse. When conduction system failures occur, the AV trigger signal arriving from the atrium is impacted and as a result, cardiac demand does not effectively control the cardiac output.
Prior art pacemakers attempt to replace the defective conduction system by establishing an artificially fixed heart rate to control a pulse generator for stimulating the ventricles. The heart rate is set sufficiently high to supply enough cardiac output to allow body maintenance as well as enough reserve to allow useful work. So-called P-synchronous prior art pacer systems attempt to use the AV trigger from the atrium to alter the heart rate so as to allow the cardiac output to better match cardiac demand. P-synchronous pacer devices have not been altogether successful in that the atrial electrical activity for a cardiac system, while containing the information concerning cardiac demand, is not a precise system and like most biological systems, it is very difficult to measure atrial trigger signals with a high degree of accuracy and reliability. Most prior art pacemaker systems, such as the P-synchronous system, attempt to duplicate the very complex function of the cardiac electrical system. However, this tends to duplicate the very same problems that the P-synchronous system is intended to correct, primarily because of improper and imperfect signal sensing, detecting, processing and controlling.
In accordance with the teachings of the present invention, the problem of defective conduction of atrial trigger signals to the ventrical is approached from the standpoint of correcting the intent of the cardiac output and cardiac demand relationship instead of attempting to reproduce the rather complex control system within the heart itself. In that the atrial electrical activity contains the information concerning cardiac demand and since this information is directly related to the P-wave rate within this activity, in accordance with the present invention, the ventricular pacing rate is controlled as a function of detected changes of the P-wave rate.